test: cover partial claim accumulation

[bobthecomputer]

Summary

• advance ledger time through ten equal vesting intervals
• claim exactly 10 percent at each interval
• verify cumulative claimed and vested values after every claim
• verify zero immediate claimable balance and both token balances after every step

Closes #15

Validation

• targeted partial-claim test: passed, 1 test
• full contract suite: passed, 5 tests
• release library check: passed
• git diff --check: passed

The test run used an ignored local lockfile resolving ed25519-dalek 2.2.0 because the repository does not commit Cargo.lock and the latest unconstrained resolution is incompatible with Soroban 22. No dependency files are included in this PR.